Antibodies to Epstein-Barr virus and other common viruses (influenza, para-influenza, rubella, measles, other herpes viruses) will be studied in lymphatic tumors, nasopharyngeal cancer, Boeck's sarcoid, systemic lupus erythematosus, and immune complex diseases. The emphasis will be on cancers and chronic diseases of unknown cause in which a defect in cellular immunity may be present. The tests will be carried out intermixed with healthy controls as well as with other types of chronic diseases. When elevated antibody titers are found their specificity and selectivity will be determined, the presence of general vs. selective hyperreactivity analyzed, and the existence of a possible defect in cellular immunity explored by skin and lymphocyte tests.